Lawyers advising thousands of plaintiffs with economic and medical damage claims from the 2010 Gulf oil spill say thousands will opt out of a complex settlement deal by Thursday's deadline, but parties that forged the agreement are confident enough will stay for the deal to work.

"I think there is very little chance that the deal will fall apart based on the number of opt-outs," said Steve Herman, a lead attorney for the Plaintiffs' Steering Committee, the court-appointed representative group for individual plaintiffs. "Compared to the thousands and thousands of claims that have been filed, and recognized, the number of opt outs is very low."

Houston lawyer Brent Coon, who says he represents about 15,000 spill claimants, said that about 10,000 will opt out - including many who aren't sure if they're eligible under the settlement. Chris Dean, another Houston lawyer who represents about 35 clients with complex business claims, said about a fourth of them are opting out - more than he expected.

The 1,200-page proposed settlement offers complex formulas to determine economic and medical compensation for thousands of fishermen, hotels and other individuals and businesses affected by the disaster.

"If they paid based on the claim calculations, they would be all right, but the big question is - are they really going to do that, or are they going to nickel and dime like they have in the past?" said Dean, referring to an earlier claims process that many spill victims found cumbersome and unfair.

The deal on the table now was negotiated by BP and the steering committee, and includes the stipulation that eligible claimants who do not want to participate must notify the court by midnight Thursday.

Those who opt out can pursue individual lawsuits. Those who stay in will be bound by the settlement.

Some claimants say the deal is so complex that they can't estimate their claim's potential value, so they have to decide whether to participate without knowing whether they will be adequately compensated.

The Deepwater Horizon Claims Center, which was established by a court-order to administer the settlement, has made about 11,400 settlement offers to claimants totaling more than $860 million since it opened at the beginning of June, said administrator Patrick Juneau. It has received about 75,000 claims and 2,300 opt-outs through Wednesday afternoon, he said.

U.S. District Judge Carl Barbier has given the deal tentative approval and is supervising the claims center.

Barbier is overseeing the tangle of litigation that began when BP's Macondo well blew out in 2010, destroying the Deepwater Horizon drilling rig, killing 11 workers and dumping millions of gallons of crude into the Gulf of Mexico.

He and others involved in the case have said it is typical in a class-action settlement not to receive an offer until after a settlement has been finalized.

"The class action has hundreds of different ways of calculating different losses," Coon said. "Some losses end up being treated favorably and some don't. It will take a crystal ball to some degree."